The present invention relates to a method, apparatus and system for monitoring hardening forecasting strength of chemically active materials, and, more particularly, to a method, apparatus and system for non-destructive forecasting of concrete strength, at an early hardening stage.
As used herein throughout the specification and in the claims section below, the phrase “chemically active material(s)” includes cementitious materials, such as, but not limited to, cement paste, mortar, concrete, lime, gypsum, clay and the like that undergo a curing process when hardening.
A chemically active material often needs to be analyzed so as to determine the structural properties parameters, particularly strength and other physical-mechanical properties of the final cured product, such as its potential for shrinkage. The final strength of a chemically active capillary-porous material is determined by the properties of the initial raw materials, mixing and compacting conditions, and specific composition such as, but not limited to, mineral binder-to-aggregate ratio, water-to-cement ratio, water-to-aggregate ratio and the like [Neville A. M., “Properties of concrete,” Longman Scientific & Technical, 1981].
Traditional prior art methods for testing the strength of concrete typically require 28 days to complete. The builder usually does not or cannot delay construction 28 days to receive the test results. Rather, the construction usually continues in the hope that the concrete is sound. If in the final analysis, the concrete does not meet the standards, the building may have to be reinforced or even torn down, perhaps incurring major additional costs.
Improvements in cement production and prediction of concrete performance properties require the application of material science. Methods of predicting the final strength of concrete while hardening have been developed over the years, based on theoretical and experimental investigations of the crystallization strengthening laws and the properties of chemically active material [to this end see, e.g., Abrams D. A. “Design of concrete mixtures,” Bull. No. 1, Sruct. Mater. Lab., Lewis Inst., Chicago, 1918; Powers T. C., “Structure and physical properties of hardened Portland cement paste,” J. Amer. Ceramic. Soc., 41, 1958, pp. 1–6; Roy D. M. and Gouda G. R., “Porosity-strength relation in cementitious materials with very high strength,” J. Amer. Ceramic. Soc., 53, No. 10, 1973, pp. 549–550; Sheikin A. E., Chekhovsky I. V. and Brusser M. I., “Structure and properties of cementitious concrete,” Stroyizdat Press, Moscow, 1973 (in Russian)].
In modern plants, the preparation of a chemically active material is typically fully automated and computer-controlled. High quality and reliability of the technological equipment and control systems is crucial for providing high quality and stable final product. With respect to concrete, for example, it is desired to optimize the process such that the strength of the concrete, 28-days from preparation is maximal.
One method to predict the concrete strength [King J. W. H., “Further notes on the accelerated test for concrete,” Chartered Civil Engineer, London, May 1957, pp. 15–19] includes warming a 6-hour age concrete to 200° F. (93° C.) and extrapolation of the obtained results 7 days and 28 days.
Also known [Y. Ono, “Microscopic observation of clinker for estimation of burning condition, grindability and hydraulic activity,” Proc. 3d Intern. Conf. Cem. Microscopy, Houston, 1981; Sinha S. K., Rao L. H. and Akhouri P. H., “Rapid estimation of the 28-day compressive strength of clinker by optical microscopy,” Proc. 13th Intern. Conf. Cem. Microscopy. ICMA, Florida, April 1991] are attempts to predict the 28-day strength of cement proceeding from the measurement results of Portland cement clinker crystals by means of an electronic microscope.
The above and other prior art methods are expensive and complicated, and require a well equipped laboratory and a highly trained material scientist or technician.
The hardening process of a chemically active material can be considered as a series of consecutive transitions between different states of the material.
Initially, the material is a compaction structure whose physical and mechanical properties are determined mainly by compressive actions of capillary pressure on “water-air” boundaries. This state is characterized by an intensive development of the chemical reactions, such as hydration and hydrolysis and formation of gel (the term gel has been introduced into the scientific practice in conjunction to cementitious materials by T. Powers in an article entitled “The non-evaporable water content of hardened Portland cement pastes,” published in ASTM Bulletin, 1949, No. 158, and was further used by A. Neville in an article entitled “Properties of concrete,” published by Longman Scientific & Technical, 1961).
In a second state following the initial state, the material develops a coagulation structure, which is a capillary-porous colloidal body having chemically active water-silicate dispersions.
In a third state the material develops a colloidal-crystalline structure, which is a quasi-solid capillary porous body. In this state the gel begins to age and crystalline structures are formed.
In a fourth state, the crystalline structures condensate, and the material has a solid capillary-porous body whose conditions are determined by the laws governing the interaction of particles and particle aggregates in the solid phase.
At any given state, the chemically active material has a poly-dispersed structure of a moist capillary-porous body. The liquid phase of the material is therefore an informative component indicative of the porosity of the material and therefore of its strength. Water (both in a liquid and gaseous form) is always in a state of thermodynamic equilibrium with the porous solid phase with which it interacts. Thus, the properties of water are changing in strict accordance with structure formation and consequently with the strength growth of the hardening material. To this end, see, for example, Shtakelberg D. I. and Sithcov M. M., “Self-organization in disperse systems,” Riga, “Zinatne” Press, 1990; and Shtakelberg D. I., “Thermodynamics of water-silicate disperse materials structure-formation,” Riga, Zinatne, 1984; and Neville M. “Properties of concrete,” Longman Scientific & Technical. NT., 1988.
The duration of the above hardening process is typically rather long. For example, in cementitious materials typical duration of hardening is of order of one month, at which time the cement passes through all the above states and becomes a solid structure of a given compressive strength.
Due to the long duration of the hardening process, prior to reaching the final strength, the chemically active material undergoes many complicated physical and chemical processes, which can essentially affect its physical properties. It is recognized that any change, deviation and non-observance of the technological regulations during preparation of the chemically active material, such as ready-mixed or pre-cast concrete, may irreversibly reduce the properties (e.g., strength) of the final product. Reasons for poor final product quality include unexpected replacement of material suppliers, improper operation of the equipment or failure thereof and the like.
Hardening and strengthening of chemically active material is initiated immediately once the compaction for a particular application is completed. However, many additional processes, affecting the final quality can takes place. For example, in case of concrete, the transportation of the mix from the manufacturing plant to the building site typically occurs between the preparation of the mix and the compaction thereof. Although during transportation the concrete mix is in a continuous motion inside a rotating drum so as to prevent setting or hardening, it is known that the final properties of cementitious products made after a prolonged transportation of the mix are different from the properties of the same products when made of a freshly prepared mix.
Prolonged transportation of the mix naturally extends the period in which chemical reactions such as hydration and hydrolysis occur. Thus, upon arrival to the construction site different transport durations result in different initial states for the hardening and strengthening processes evolve.
Other factors which are known to alter the hardening process include, chemical additives of various functional purposes, temperature conditions during hardening, curing conditions of the freshly formed concrete, non-homogeneity of the mix, complexity and duration of the manufacturing process and the like.
It is therefore recognized that an optimal final product requires a continuous and operative monitoring during manufacturing and throughout the hardening and strengthening stages.
Although the nature of hardening and strengthening processes is, in principle, deterministic, prior art methods and apparati fail to provide an accurate and reliable technique for forecasting strength the strength of the final product by monitoring the hardening and strengthening stages of the mix.
There is thus a widely recognized need for, and it would be highly advantageous to have a method, apparatus and system for monitoring hardening and forecasting strength of cementitious material, devoid of the above limitations.